Click above to get retro games delivered to your door ever month!

  Every PC and PS/2 has a video subsystem responsible for producing the
  image that appears on the screen. At the heart of the video subsystem is
  the special-purpose circuitry that must be programmed to generate the
  electrical signals that control the video display. Most members of the
  PC/XT/AT family require you to install a display adapter, a special video
  circuit board that plugs into one of the computer's expansion slots. On
  the other hand, all PS/2s are equipped with built-in video circuitry and,
  therefore, require no display adapter.

  The video circuitry consists of a group of interrelated components that
  control signal timing, colors, and the generation of text characters. All
  IBM video subsystems have a video buffer, a block of dedicated memory that
  holds the text or graphics information displayed on the screen. The video
  subsystem performs the unique task of translating the raw data in the
  video buffer into the signals that drive the video display.

  The various video subsystems used in PCs and PS/2s all evolved from the
  two video adapters originally released by IBM for the PC: the Monochrome
  Display Adapter (MDA) and the Color Graphics Adapter (CGA). IBM later
  released its Enhanced Graphics Adapter (EGA), a more powerful successor to
  the MDA and CGA.

  When the PS/2s appeared, IBM introduced two more video subsystems: the
  Multi-Color Graphics Array (MCGA), built into the PS/2 models 25 and 30,
  and the Video Graphics Array (VGA), built into the PS/2 models 50, 60, and
  80. At the same time the PS/2s appeared, IBM introduced a VGA adapter that
  can be used in the PC/XT/AT family as well as in the PS/2 Model 30.

  We'll be discussing all five of these IBM subsystems--MDA, CGA, EGA, MCGA,
  and VGA--in this chapter. Although clear differences in hardware design
  exist between the various video subsystems, their strong family
  resemblance should encourage you to consider what they have in common
  before worrying about the differences between them.

  Most of the five video subsystems can be programmed into two fundamentally
  different modes, called text mode and graphics mode by IBM. (The lone
  exception is the MDA, which operates only in text mode.) In text mode you
  can display only text characters, though many of these characters are
  suitable for producing simple line drawings. (See Appendix C for more on
  characters.) Graphics mode is mainly used for complex drawings but you can
  also use it to draw text characters in a variety of shapes and sizes.

  The CGA can operate in both text and graphics modes to produce drawings
  and characters in several formats and colors. By contrast, the MDA can
  operate only in text mode, using a stored set of ASCII alphanumeric and
  graphics characters and displaying them in only one color. The MDA works
  only with the IBM Monochrome Monitor (or its equivalent) while the CGA
  must be connected to either a direct-drive or a composite color monitor.
  (See page 74 for more on monitors.) Many business and professional users
  prefer a monochrome display to a color display because a monochrome screen
  is easier on the eyes and less expensive than an equivalent color display.
  But in choosing monochrome, they sacrifice color, a valuable asset for any
  computer display.

  The MDA's most obvious drawback is its inability to display images in
  graphics mode. For this reason, PC/XT/AT users who prefer a monochrome
  display, yet need to view graphics, must turn to an EGA or to a non-IBM
  adapter like the Hercules Graphics Card, which emulates the MDA's text
  mode but supports a monochrome graphics mode as well.

  Roughly two-thirds of all PCs are equipped with the standard MDA and
  therefore have no graphics or color capability. While there are real
  advantages to using color and graphics, most PCs get along nicely without
  either. Although the clear trend is toward higher-performance video
  subsystems that can display graphics as well as text, keep in mind as you
  plan computer applications that many PCs display text only.

  The best way to understand the video capabilities of the PCs and PS/2s is
  to cover the features that their various video subsystems have in common.
  As we go along, we'll point out the differences and improvements that
  distinguish the newer and more complicated subsystems (EGA, MCGA, and VGA)
  from their predecessors (MDA and CGA).